The present invention relates to a method of producing a valve seat for a suction-exhaust valve of an internal combustion engine, and particularly to a method of producing a compound valve seat formed by a powder metallurgy technique.
Special materials are typically used for valve seats because the valve seat is required to withstand abrasion due to beating against the valve as well as corrosion/wear due to high temperatures. A sintered alloy is generally used because of its superior producibility. As the sintered alloy valve seat, generally, stellite powder or ferro-molybdenum powder for forming hard particles is mixed with base iron powder and then sintered to obtain a material in which hard particles and sinter holes are dispersed in the iron-base sintered alloy base, which material is superior in wear-proof characteristics. However, the sintered alloy contains a large amount of hard particles as mentioned above, and therefore a large amount of valuable elements such as cobalt and tungsten are required, so as to make the product expensive. Accordingly, the part of the cylinder head side of the valve seat in which wear-proofing is not required may be made of a sintered alloy requiring little additional elements, having a predetermined strength, and having a thermal expansion factor and a sintering shrinkage factor both of which approximate those of the sintered alloy at the valve seat side, so as to fabricate a compound valve seat.
As shown in FIG. 1, the compound valve seat is made of a first sintered alloy 11 forming the cylinder head side and a second sintered alloy 12 forming a valve seat surface side, produced in a manner such that after a first powder for forming the first sintered alloy has been charged into a metal mold, a second powder for forming the second sintered alloy is charged over the first powder and the two layers of stacked powder are integrally compressively molded. In this method, the boundary plane between the first and second powder is indefinitely formed to cause large variations in the shape thereof, so that it is necessarily required to make the quantity ratio of the second powder, which attains the function of the valve seat per se, to the first powder large.
In order to reduce the quantity of the second powder, to attain resource savings and cost reduction which are the main objects of the compound valve seat, a valve seat as shown in FIG. 2 has been conventionally proposed. In the valve seat of FIG. 2, the second sintered alloy 12 is disposed only at the position including the valve seat surface. Such seat structure is disclosed, for example, by Japanese patent publication No. 39166/1976. To produce the compound valve seat shown in FIG. 2, it is necessary to prepare to a press-molding machine provided with two coaxial lower punches, resulting in problems in that not only is the operation of the molding machine complicated, but the lifetime of the lower punches is short and the practical application of the relative position control of the two lower punches is difficult and requires a high precision technique.
Further, in producing a compound sintered member, a method has been proposed in which the first powder is charged in advance and preparatively compressively molded, and then the second powder is charged, as disclosed, for example, by Japanese patent application Laid-Open No. 11280/1974 and Japanese patent publication No. 43963/1979. This method is, however, generally used for producing a valve seat constituted by two laterally arranged inner and outer layers, and therefore it is difficult to employ this method in producing a compound valve seat constituted by two vertically disposed upper and lower layers.
That is, when a space for charging the second powder thereinto is formed around the preparatory green compact preliminarily formed with the first powder, there is a risk of deformation or deintegration of the preparatory green compact because of the spring-back force of the pregreen green compact per se. Further, since the preparatory compact which has been preliminarily compressively formed is pressed against a die and/or a core rod by the spring-back force thereof, it is difficult to form a predetermined space above the compact by moving the die/core-rod and therefore there is a limitation in the form of the moldings.
Further, since a valve seat has a shape such that, as shown in FIGS. 1 and 2, the valve seating surface is largely chamfered, a conically-shaped protrusion corresponding to the shape of the valve seat surface is provided on an upper or lower punch of the press-molding machine. When the protrusion is provided on the upper punch, however, powder D of a quantity corresponding to the volume of the protrusion 51 may overflow onto the top surface of the die 2 as shown in FIG. 3, resulting in the mixture of different kinds of powders in the manufacture of the compound valve seats, and therefore the protrusion is usually provided on the lower punch. In the case where the protrusion is formed on the lower punch, however, the press-molding machine can only produce such a compound valve seat as shown in FIG. 1, which is not effective in reducing the quantity of the second sintered alloy.